The preparation of hydrocarbons from a H.sub.2 /CO mixture by contacting this mixture with a catalyst at elevated temperatures and pressures is known in the literature as the Fischer-Tropsch synthesis.
Catalysts often used for this purpose comprise one or more metals from the iron group supported on a carrier.
A very active catalyst system consists of cobalt supported on a silica carrier and, as promoter, zirconium.
The Fischer-Tropsch catalysts are suitably prepared by combining the active ingredients and the carrier, e.g. by precipitating the metal on the carrier from an aqueous solution of the metal or, preferably, by impregnating the carrier material with a compound of the metal in the presence of a liquid. Moreover, kneading techniques may be applied, especially if it is intended to prepare catalysts to be used in slurry reactors. In all preparation procedures, the liquid is removed from the composition followed by calcination and reduction.
In the calcination step, the impregnated metal compound is converted into one or more metal oxides. In order to be catalytically active in the conversion of carbon monoxide and hydrogen, it is necessary that a substantial amount of the metal oxide(s) be reduced under formation of the metal. The reduction is carried out in the presence of hydrogen usually at elevated temperatures, e.g. in the range of 200.degree.-300.degree. C. It has been observed that in some cases in the compositions to be used as catalysts, metal hydrosilicates occur, which do not exhibit catalytic activity. Because the reduction of these hydrosilicates is laborious, or requires severe conditions, it has been investigated whether the formation of the said hydrosilicates can be minimized, or that their removal can be facilitated.
It has now been found that by incorporating a specific further metal promoter in the catalyst the reduction step is considerably facilitated.